Buckles for safety belt systems are known in numerous constructions. A design has proved itself in which an insert path for the insert tongue is formed in the loadbearing housing of the buckle. The locking bar which is displaceably guided or pivotally mounted on the housing transversely of the insert path cooperates with a detent opening of the insert tongue. A blocking member guided displaceably parallel to the insert path in the housing holds the locking bar in its locking position as long as a release button likewise guided displaceably parallel to the insert path in the housing is in its rest position. This release button is connected to the blocking member for moving the latter on actuation into a release position in which the locking bar comes free of the detent opening of the insert tongue.
The use of such a buckle in safety belt systems having a belt pretensioner is not problematical when the pretensioner force engages for example the spool of the belt retractor. Belt pretensioners have also been proposed which act between the buckle and its attachment point on the vehicle body or a vehicle seat. Such pretensioners shorten the distance between the attachment point of the buckle and, the buckle itself by a few centimeters, for example 10 centimeters, by moving the buckle towards its attachment point on the vehicle. The pretensioners used in such cases have a mechanic drive with a force accumulator in the form of a tensioned spring which is released by a sensor responsive to inertia forces and effects a belt tightening operation when required.
The force required for the tightening of the belt can be generated mechanically by means of a strongly dimensioned spring or pyrotechnically. If the tightening force is of adequate magnitude, in particular when using pyrotechnical pretensioners, in certain cases when using a buckle of the type set forth above, an unintentional release of the insert tongue from the buckle can occur at the end of the pretensioning stroke.
This unintentional release of the insert tongue is due to the mass inertia of the release button and any components engaging thereon because the release button tends at the end of the pretensioning stroke to continue its movement in the direction of the stroke, which corresponds to the actuating direction of the release button. It has therefore already been proposed to prevent this continued movement of the release button under the influence of inertial forces by using compensating masses or blocking pawls. With moderate tightening forces as generated by mechanical pretensioners such solutions are perfectly practicable. However, with the extremely high tightening forces which can be generated by pyrotechnical pretensioners all the known solutions prove to be useless because either they respond too slowly and thus cannot prevent an unintentional release of the insert tongue or they do not stand up to the extreme mechanical stresses.
If the buckle is provided with a pawl which at the end of the pretensioning stroke becomes active through mass inertia to prevent the movement of the release button in the actuating direction, then said pawl represents a constructional component which will never become active during the life of the buckle. It is only during a pretensioning operation, which possibly might not occur until the buckle has been used for ten years, that the pawl must move out of a rest position under its mass inertia into a locking position. In its rest position it is generally held by a spring. Now, there is no excluding the possibility that in the course of the long use period of the lock impairments of the functionability of the pawl occur. For example, by soiling or penetration of extraneous particles it can be prevented from moving out of its rest position into its blocking position.